1. Field of the Invention
The present invention relates generally to a visible light communication method and apparatus, and more particularly to a method and an apparatus for visible light communication, which can secure continuous service during movement between cells of a visible light communication service using a Time Division Multiplexing (TDM) scheme.
2. Description of the Related Art
Due to improvements in light emitting efficiency and decreases in price for Light Emitting Diodes (LEDs), the LED has been generally introduced into the market of general lighting, which includes the florescent lamp and the incandescence lamp, as well as the market of special lighting, which may include a portable device, a display, an automobile, a traffic light, and an advertising board. Further, due to recent trends, such as the exhaustion of frequencies in the Radio Frequency (RF) band, crosstalk probability between several wireless communication technologies, increased demand for communication security, and the advent of an ultra high-speed ubiquitous communication environment of 4G wireless technology, interest in optical wireless technology compatible with RF technology has increased.
Visible light communication, which transfers information using visible light, is advantageous in that it can accurately recognize a reception range of information because it is possible to view a destination of light or a progress direction of light. Visible light communication is also safe, has a broad use band, and can be used without restriction. Therefore, visible light communication is reliable in a security aspect and can be driven with low power, which is advantageous in a power consumption aspect. In this respect, visible light communication can be applied to a hospital and an airplane where the use of the RF band is restricted, and also can provide additional information service using an electric signboard. Visible light communication is described with reference to the drawings below.
FIG. 1 is a diagram illustrating a visible light communication system using a general Visible Light Communication (VLC). The general VLC system includes a light source 10 functioning as a lighting formed with an LED or a Laser Diode (LD), and transmitting/receiving data using visible light. The general VLC system also includes a VLC terminal 20 including a visible light receiving/transmitting module, which performs data transmission/reception with the light 10. The VLC terminal 20 includes a mobile terminal, such as a mobile phone or a PDA, or a fixed terminal, such as a desktop computer. Further, the VLC can be linked with a communication system using another wired/wireless communication medium, so that it can be used more efficiently.
When it is necessary to provide service using the VLC to a broad space, the multiple light sources 10 are installed within a corresponding space according to a service range of each of the light sources 10. For example, as illustrated in FIG. 2, the multiple light sources 10 can be installed in a ceiling of a single room. Each of the light sources 10 can be installed in a lattice-type pattern with a predetermined interval, considering a service area within which the single light source 10 can provide the light communication service.
As illustrated in FIG. 2, when the multiple light sources in a cell unit are installed with the interval in the general VLC system, the actual service areas of each of the light sources 10 are overlapped, boundaries of the service areas are in contact with each other, or a non-service area is present between service areas of each of the light sources. The ideal state is when the boundaries of the service areas are in contact with each other, but it is difficult to actually realize this state. In the general service areas according to the multiple light sources, an overlapped area in which the service areas are partially overlapped exists or a non-service area exists between two service areas.
However, when the multiple light sources having the partially overlapped service area provide different kinds of services, data transmitted by the two services collide in the overlapped region, making it impossible to provide the regular service.
In order to provide the regular service in the overlapped area, different time slot resources are used in adjacent cells, and the adjacent cells avoid the overlapped portion in time during the communication, thereby reducing inter-cell interference.
When a cell size increases in the VLC, the interference between the adjacent cells or the support of the terminal mobility is effective, but the entire efficiency of the system is deteriorated. Further, when a cell size decreases, the efficiency of the system is improved, but the inter-cell hand-off and the interference between adjacent cells increase.
Further, when the continuous mobility is decreased, the continuous mobility can be secured in using the time resource which is the same as the time slot resource used in a previous cell. However, when another user uses the time resource, which has been used in the previous cell, in a mobile cell, it is difficult to support the continuity.